Recently, since their optical property and mechanical property are excellent, a film or plate made of a polycarbonate resin (hereinafter sometimes called as just polycarbonate) have been widely used in the variety of the usages. Such polycarbonate is often applied with the surface treatment, not solely used. For example, as for the DVD (Digital Versatile Disc), a recording layer is formed on one surface of two sheets of polycarbonate, and then they are laminated by using an UV curable adhesive. Further, the next generation optical disc (Blu-ray Disc (trade name)), which uses a blue short wavelength laser as the recording/replaying medium, has been invented. As shown in FIG. 4, since an objective lens having a high resolution and short focus length is used for such next generation optical disc 50, a laminated construction of a polycarbonate sheet (thickness: 1.1 mm) 52 having a recording layer 54 with a light transmission protective film 58 consisting of a thin polycarbonate film (thickness: 0.1 mm) by using an adhesive layer 56 is proposed.
Therefore, when the specific treatments are applied to such polycarbonate film or plate, a protective film is laminated onto the surface of polycarbonate or the other protective film is co-winded with polycarbonate in order to prevent scratch and to provide the smoothing ability in each step.
For example, JP2001-303005A has disclosed the polyethylene film having the specific pressure sensitive adhesive layer in order to provide a surface protecting film for a polycarbonate resin plate having the small adhesive strength change even heated at high temperature as well as a polycarbonate resin plate in the heating process and the sufficient initial adhesive strength at the normal temperature.
More concretely, a surface protecting film for a polycarbonate resin plate was disclosed in that a pressure sensitive adhesive which comprised the specific ethylene-α-olefin copolymer made from a polymerization method using a single sight catalyst to obtain the proper initial adhesive strength, and the specific low density polyethylene resin made from a polymerization method using a Ziegler catalyst to obtain the proper stable adhesive strength with times in a fixed additional ratio respectively, was laminated onto the polyethylene film by a co-extruding method.
Here, for the disclosed surface protecting film for a polycarbonate resin plate, it is preferable that the initial peel adhesive strength should be a value within about 100 to 500 mN/25 mm due to preventing the self peeling from the surface protecting film as well as being easy to peel off such surface protecting film when it is taken away from a polycarbonate resin plate.
Further, JP1-242679 A has disclosed the making method for a surface protected article in that a surface protecting film comprising thin plastic or metal substrates and an UV (ultraviolet) curable adhesive was laminated in the manner that it was easy to peel off from an objective substrate.
More concretely, the making method for the surface protected article was disclosed in that the UV curing process for an UV curable adhesive provided on a surface protecting film was carried out in considering of the material and surface roughness of the objective substrate and the like, due to adjusting the adhesive strength of the UV curable adhesive and then such surface protecting film was laminated onto the objective substrate via the UV cured adhesive.
Further, JP2001-243659 A has disclosed a plastic film winding obtained from laminating a surface protecting film which is for an optical disc as a transparent protecting film with a polyethylene terephthalate film without an adhesive and winding up them.
More concretely, the polycarbonate film which was for laminating an optical disc substrate made from a polycarbonate resin and had the specific thickness, surface roughness and the like, and a polyethylene terephthalate film were laminated without an adhesive, and then the plastic film winding were made from the above laminate by using a winding machine and the like.
However, it was necessary to apply the considerable strong tension when the surface protecting film for a polycarbonate resin plate was laminated, since the mechanical property of such surface protecting film having the substrate consisting of a polyethylene film disclosed in JP2001-303005A was insufficient and the wrinkle were easily formed.
Therefore, when the polycarbonate film laminated with such surface protecting film was stored for a long period, there was a drawback that strong curling of the polycarbonate film occurred, due to the strong residual stress of the surface protecting film and therefore processing of the polycarbonate film deteriorated. Also, when the polycarbonate film laminated with such surface protecting film in the winded roll was stored for a long period, there were drawbacks that the winding compaction and the wrinkle deformation occurred and the polycarbonate film laminated with such surface protecting film could not be practically used.
Also, the making method for the surface protected article in that the UV cured pressure sensitive adhesive was used in considering of the surface property of the objective substrate, which was disclosed in JP1-242679A, mainly considered the surface protecting film for a metal plate and did not regulate the grass transition temperature and the peel adhesive strength of UV cured pressure sensitive adhesive which was not practical use for the surface protecting film for the polycarbonate film.
In addition, the property of the film substrate was not considered, only polyethylene film was described and the aforementioned curling and the winding compaction were not sufficiently prevented by using the polyethylene film.
On the other hand, the surface protecting film disclosed in JP1-242679A generally showed the higher peel adhesive strength to polycarbonate than that of the releasing film to polycarbonate film having the pressure sensitive layer. Thus, as shown in FIG. 9(a), the peel adhesive strength of the interface “A” between the surface protecting film 10 consisting of the film substrate 12 and adhesive layer 14, and the polycarbonate film 58 is generally higher than that of the interface “B” between the polycarbonate film 58 having the adhesive layer 56 and the releasing film 60.
Accordingly, when the surface protecting film 10 was released from the polycarbonate film 58, namely the surface protecting film 10 was released at the interface “A”, the polycarbonate film 58 having the pressure sensitive adhesive layer was released from the releasing film 60 at the interface “B”. So, as shown in FIG. 9(b), the surface protecting film 10 may be released from the polycarbonate film 58 having the pressure sensitive adhesive layer only after laminating such polycarbonate film 58 with the other substrate such as the polycarbonate substrate 52 having a recording layer 54. Accordingly, the conventional surface protecting film has drawbacks that the timing of releasing the polycarbonate film is greatly limited.
Further, the plastic film winding disclosed in JP2001-243659A, when the polycarbonate film was applied for coating applications and the like in the case of re-winding from the roll, the polycarbonate film and the co-winded polyethylene terephthalate film might be separated, and the polycarbonate film might be scratched after the application steps.
Still further, the disclosed plastic film windings has to take away the polyethylene terephthalate film in the application step of the polycarbonate film, the other film has to be winded again with the polycarbonate film or the other surface protecting film has to be laminated with it due to preventing the scratch and the like.
That is, the present invention has the following objects;    1) To provide a surface protecting film, wherein it is easy to laminate with polycarbonate, to prevent the self peeling, and to peel off from polycarbonate optionally.    2) To provide a surface protecting film, wherein the increasing peel adhesive strength (sometimes called as 180° peel adhesive strength) change to polycarbonate with times is suppressed even after storing for a long time or at a high temperature in the roll shape in the condition that the surface protecting film is laminated with polycarbonate as well as preventing the winding compaction.    3) To provide a surface protecting film, wherein it is easy to peel off the surface protecting film from polycarbonate before applying the polycarbonate laminated with the surface protecting film to the other substrate via an adhesive layer.    4) To provide a surface protecting film, wherein there is no adhesive transfer to polycarbonate, when it is laminated with polycarbonate.